Field of the Invention
The invention relates to a method for casting cast parts in which a molten metal is poured into a casting mould which encloses a cavity forming the cast part which is to be produced, wherein the casting mould, designed as a lost mould, consists of one or more casting mould parts or cores. The casting mould parts or casting cores are thereby formed of a mould material which consists of a core sand, a binder and, optionally, one or more additives for adjusting particular properties of the mould material.
Description of Related Art
In conventional methods of this kind, the casting mould forming the cast part is usually provided first, the casting cores and mould parts of which have been prefabricated in separate working operations. The casting mould can thereby be composed, as a so-called “core package”, of a plurality of casting cores. Equally, it is possible to use casting moulds which are, for example, composed of only two mould halves consisting of mould material, in which the mould cavity forming the cast part is formed, wherein here too mould cores can be present in order to form recesses, cavities, channels and similar in the cast part.
Typical examples of cast parts which are produced by means of a method according to the invention include engine blocks and cylinder heads. For larger engines subject to high loads, these are manufactured of cast iron by means of sand casting.
In the field of iron casting, quartz sands, mixed with bentonites, lustrous carbon formers and water are usually used as mould material for casting mould parts forming the outer closure of the casting mould. The casting cores forming the interior cavities and channels of the cast part are, in contrast, usually formed of commercially available core sands, which are mixed with an organic or inorganic binder, for example with a synthetic resin or water glass.
Irrespective of the type of core sands and binders, the basic principle behind the manufacture of casting moulds formed of mould materials of the aforementioned type is that, after forming, the binder is hardened by means of a suitable thermal or chemical treatment, so that the grains of the core sands adhere together and the stability of form of the relevant mould part or core is ensured over a sufficient duration.
Particularly when casting large-volume cast parts made of cast iron, the internal pressure exerted on the casting mould following the pouring of the molten metal can be very high. In order to absorb this pressure and reliably prevent the casting mould from bursting, either thick-walled large-volume casting moulds must be used or supporting structures must be used which support the casting mould on its outer side.
One possible form of such a supporting structure consists of an enclosure which is placed over the casting mould. The enclosure is usually designed in the form of a jacket which surrounds the casting mould on its peripheral sides but which has on its upper side a sufficiently large opening to allow the melt to be poured into the casting mould. The enclosure is thereby so dimensioned that, after it is placed in position, a filling space remains between the inner surfaces of the enclosure and the outer surfaces of the casting mould, at least in the sections decisive for the support of the casting mould. This filling space is filled with a free-flowing filling material, so that a support of the relevant surface sections over a wide area by the enclosure is guaranteed. In order to ensure as even as possible a filling of the filling space, an equally even contact between the casting mould and the filling material and a correspondingly even support of the fragile mould material, as a rule fine-grained, free-flowing filling materials such as sand or steel shot are used which have a high bulk density. After filling, the filling material is additionally compacted. The aim here is to create the most compact possible filling mass which, acting like an incompressible monolith, ensures the direct transmission of the supporting forces from the enclosure to the casting mould.
The molten metal is poured into the casting mould at a high temperature, so that the casting mould parts and cores of which the casting mould is composed are also heated strongly. Consequently, the casting mould begins to radiate heat. If the temperature of the casting mould exceeds a certain minimum temperature, then the binder of the mould material begins to vaporise and combust, releasing further heat. This causes the binder to lose its effect. As a result of this decomposition of the binder, the cohesion of the grains of the mould material of which the casting mould parts and cores of the casting mould are made is lost and the casting mould and its parts and cores made of mould material collapse into individual fragments.
It is known in practice that this effect can be used for the demoulding of the cast parts from the casting mould. Thus, heat treatment methods for cast parts are for example known from EP 0 546 210 B2 or EP 0 612 276 B2 in which the casting mould together with the cast parts are, in a continuous process sequence, transferred from the casting heat into a heat treatment furnace. While passing through the furnace, the casting mould and the cast parts are exposed over an adequately long duration to a temperature at which the condition of the cast parts is achieved which is the objective of the heat treatment. At the same time, the temperature of the heat treatment is so selected that the binder of the mould material decomposes. The fragments of the casting mould consisting of mould material which then automatically fall away from the cast part are collected in a sand bed in the heat treatment furnace itself. They remain there for a certain period in order to further encourage the disintegration of the fragments of the casting mould parts and cores. The fragmentation of the mould material falling from the casting mould can be supported in that the sand bed is fluidised by blowing in a hot gas flow. The sufficiently disintegrated mould material fragments are finally fed to a processing facility in which the core sand is reclaimed so that it can be used for the manufacture of new casting mould parts and cores.
The known procedure for the demoulding and processing of the casting moulds required for the casting of cast parts has proved effective in practice in the casting, in large quantities, of parts for internal combustion engines made of aluminium. However, this requires a furnace of considerable construction length and a handling of the casting moulds and cast parts which in the case of high-volume parts or casting moulds, requiring additional support through an enclosure of the type described above, proves complicated. This applies in particular to cast parts which are to be manufactured in small and medium-sized quantities from cast iron.